Humans have evolved to become little more than brains (ala krang from TMNT), and as such, required to be placed inside fully mobile mechanical robots to give them the freedom to continue pursuing the outer galaxies, and conquering THE NINJA TURTLES (or the far reaches of space)

Are you in a hurry? No, or you wouldn't be posting here over several days.
Is it Japanese? No, "carmine" can't be Japanese.
Is it a fantasy game? No.
Is it a sequel? No.
Is it about two people teaming up? No. At least, not from the screenshot you posted. Could change.
Is it a sci-fi game? Yes, definitely.
Use a single word that could pass off as the name of a heavy metal band. Hmm... How about Technetium?
Does it sorta make sense? No, not really.
Slap it on the box!

Now all you need is a backstory to explain how an artificial element used for radioactive tracing in medicine relates to giant mechs!

Are you in a hurry? No, or you wouldn't be posting here over several days.
Is it Japanese? No, "carmine" can't be Japanese.
Is it a fantasy game? No.
Is it a sequel? No.
Is it about two people teaming up? No. At least, not from the screenshot you posted. Could change.
Is it a sci-fi game? Yes, definitely.
Use a single word that could pass off as the name of a heavy metal band. Hmm... How about Technetium?
Does it sorta make sense? No, not really.
Slap it on the box!

Now all you need is a backstory to explain how an artificial element used for radioactive tracing in medicine relates to giant mechs!

Hah ya know what's funny is at first I was like "Technetium" is a pretty plausible game name... let me google for it.. then.. damnit there is a game company called Technetium Games

I've always found mechs fun and fascinating, but I've never played a mech game. The idea of using mechs for combat is exciting, but somewhat absurd. Any battle fought with them would be short and one-sided, at least until the other side acquired some portable missile launchers. Like most military machinery, they represent concentrated targets, only more-so: the most elaborate, expensive machine you could imagine, put right up in harm's way.

So you can either play that up, mitigate it, outright ignore it, or avoid it.

How much do you want to focus on the world in which the game takes place? You could try to find a motivation for why an economy might produce giant robotic vehicles. I remember in Robotech there was some narrative sense to it, even though the logic was inverted, because the enemy were giant people. (There was no clear reason why the enemy themselves rode around in walking robots, though, besides looking interesting.) I guess the Avatar motivation was similar.

Humanoid and animal forms are attractive because we understand them and they resonate with our subconscious, so if you can combine that with a semi-reasonable excuse for why a society would invest in building them, you would have the makings of a compelling scenario.

There are at least two reasons I could think of. The obvious one is work in hostile environments, especially those not suited to either wheeled vehicles or paving. They can navigate difficult terrain, endure high gravitational fields, and manipulate strangely shaped, extremely large or heavy objects in ways too awkward for conventional industrial machines and vehicles.

The other reason is aesthetic. A culture which evolved in this way due to some strange belief system. And extremely wealthy, enough to afford the materials and energy to make these robots.

You can mitigate the problems with mech warfare by inventing things that would protect the mechs and make it less likely for them all to be destroyed in a very short time. Or perhaps the world in which they operate is already extremely unequal: only one political faction has them, and they oppress everyone else. Or maybe all the other machinery was destroyed in an ongoing slow war, and mechs are all that is left: militarized industrial machines.

Every game needs a setting, although for practical purposes it may not matter whether the game is set on Earth somewhere, or on another planet in our solar system or another one. A future where interplanetary travel is common also predicates access to more energy resources (good old fusion power, or something more exotic) to power spacecraft (and factories), which, miniaturized, would also power the mechs.

Then you need to find characters and motivation for them and your player. Why am I riding around in this gigantic robot? Why am I fighting this other robot, or doing this industrial work, or exploring this strange, high-gravity world, or this dark, dangerous underground world, etc.

Of course, you could simply throw all that in the bin, and just make an aesthetically pleasing, utterly storyless combat game like this one: Hawken.